The invention relates to a metal-plastic composite material that is stretch-formable and/or deep-drawable, the use of the composite material and containers made from the composite material.
It is known to use stretch-formed, aluminium shaped-packs made from composite materials for containment of pharmaceutical products and foodstuffs e.g. as push-through packs, containers, under-seal trays, double moulds and the like. These are able to satisfy fully the high demands made with respect to protection of the contents against diffusion of oxygen, steam and light, or preventing loss of fragrance, for example, and to improve the user-friendliness of the of the packaging.
Further requirements that arise are the suitability for sterilisation, pasteurisation or for hot-filling.
It is state-of-the-art to manufacture stretch-formed containers from composite or laminate materials featuring a central aluminium layer, an outer layer of oriented polyamide and an inner layer of polyvinyl chloride. Today, in order to protect the environment, the use of polyvinyl chloride should be limited. Polyvinyl-chloride-free layers or those low in polyvinyl chloride have the advantage of producing no or only little pollution if they are energy-recycled instead of material-recycled. On disposing of laminates of this type it is also advantageous if the number of different types of material present is as small as possible.
Composite materials for packaging purposes containing no polyvinyl chloride are known per se.
Described in EP-A-0 474 587 is a composite for containers that features barrier properties. The composite features a metal layer on both sides of which is a layer of polyolefin. This composite is not able to satisfy all the properties required today
Described in DE-OS 27 57 370 is a laminate film for manufacturing deep-drawn shaped parts. The said laminate film, however, requires a specific aluminium foil.
U.S. Pat. Nos. 4,085,244 and 4,216,268 describe a laminated packaging film comprising an outer biaxially-oriented polyamide film, a flexible metal foil, a biaxially-oriented polypropylene film and an inner sealing layer for manufacturing pouches.
Because of its asymmetric make-up, viz., relatively thick inner polypropylene layer, this film is not very suitable as a stretch-form laminate. Also, because of the two classes of plastic present, this material is more difficult to recycle.
German patent document 34 36 412 describes a metal sheet which is laminated on one or both sides with a biaxially-oriented polyester film and is considered suitable for drawing cans. In order to ensure proper bonding between the metal and the polyester foil, the metal sheet must have a double layer of hydrated chromium oxide. A metal sheet of this type is complex to manufacture and chromium oxide layers are undesirable, for example in foodstuff packaging, or may be prohibited.
None of the composite materials, laminate films or laminates described above can be combined to give the optimum properties required for stretch-forming. This also applies to the properties required for protection of the environment which promote e.g. recyclability of the laminates or objects prepared therefrom, such as forms of packaging.
The packaging laminates known up to now exhibit the disadvantage of not being equally suitable at meeting all of the properties required by the packaging industry, such as flatness (rigidity) after shaping, ability to be deep-drawn or stretch-drawn, to withstand shock and/or long term cooling, vacuum conditions, long term storage and pasteurising and sterilising conditions.
A suitable composite material must assist elongation of the metal layer during the stretching process so that elongations per unit area of 80% and more can be achieved without damage occurring, such as perforation of the metal layer. Protecting the metal layer from perforation or tearing is very significant as the metal layer serves mainly as a barrier layer against inward diffusion of gases such as oxygen, penetration of steam and light, and against the outward diffusion of fragrance and aromas.
After the deep-drawing or stretch-drawing operation the composite material must capable of undergoing a shock-freezing and/or freeze-drying process without delamination and change of shape.